High speed optical scanning systems, such as precision plotters, printers and the like are well known in the art. These devices are direct imaging systems and are used to fabricate printed circuit boards (PCB) artwork and printing plate artwork by raster scanning an exposure beam across a film which is further processed into a mask. A typical system as marketed by The Gerber Scientific Instrument Company, the assignee of the present application, consists of a magnetic tape device, hard disk, computer, interactive graphics terminal, image processor, optical table having a movable write platen for positioning the substrate and a precision laser scanner. The system also includes such optics, media carriage and electronics as is necessary to directly transfer computer aided design (CAD) data to the printed circuit board artwork or transfer fonts, graphics and half toned images into printing plate artwork.
In operation, a direct imaging system is configured to receive on the write platen a planar substrate of aluminum (in the case of graphic arts) or copper clad dielectric (in the case of a printed circuit board) which has an optically sensitive photopolymer applied to its upper surface. The computer modulators the intensity of an optical beam, usually provided by a laser, to expose selected portions of the substrate. Typically, there is a second, reference beam scanned simultaneously with the exposing beam for generating a scan timing signal and for accurately controlling the position of the exposing or write beam on the substrate. A flat field scanning system is sometimes employed to focus the beams to small spots and to accomplish the simultaneous scanning of the beam across a reference mask and the substrate, respectively. Precision air bearings are often used to guide the write platen as the substrate is imaged.
The reference beam scans a precision target which typically consists of a linear array of transparent and opaque bars on a substrate (i.e. 1 mil. transparent/1 mil. opaque). After modulation by this target the light is typically gathered by a linear array of optical fibers positioned behind the reference mask and registered with the scan line. As the reference beam traverses the scan line and is mounted by the target, light enters the optical fibers and is received by one or more photodetectors. A controller uses the output signal from the photodetectors to generate a clock signal.
To extend over a typical scan line length of twenty inches, the linear array of optical fibers must comprise a great many optical fibers. Moreover, nonuniform attenuation by different fibers can cause timing errors. Known devices for generating scan timing signals in large raster scanners are therefore expensive and prone to error.